Low-motional-impedance highly-tunable I2 resonators for temperature-compensated reference oscillators

A new capacitive micromechanical resonator design optimized for high Q, low motional impedance, and large tuning range is presented. The I² resonator was fabricated using a conservative, by present standards, HARPSS-on-SOI process, and demonstrated quality factors up to 57000 in vacuum. From the designed resonators between 3MHz and 6MHz, the lowest measured impedance is 24kΩ and the largest electrostatic tuning coefficient is -10ppm/V². An oscillator interface circuit comprising of a trans-impedance amplifier and an automatic tunable charge pump providing an automatic temperature-compensating bias voltage was also designed and fabricated in a standard 0.5μm CMOS process. Preliminary tests show temperature drift reduction of a 4MHz resonator from 2400ppm to 240ppm over a 90°C range.